Construction of Hollow Co3O4@ZnIn2S4 p-n Heterojunctions for Highly Efficient Photocatalytic Hydrogen Production

Photocatalysts derived from semiconductor heterojunctions for water splitting have bright prospects in solar energy conversion. Here, a Co3O4@ZIS p-n heterojunction was successfully created by developing two-dimensional ZnIn2S4 on ZIF-67-derived hollow Co3O4 nanocages, realizing efficient spatial separation of the electron-hole pair. Moreover, the black hollow structure of Co3O4 considerably increases the range of light absorption and the light utilization efficiency of the heterojunction avoids the agglomeration of ZnIn2S4 nanosheets and further improves the hydrogen generation rate of the material. The obtained Co3O4(20) @ZIS showed excellent photocatalytic H2 activity of 5.38 mmol g−1·h−1 under simulated solar light, which was seven times more than that of pure ZnIn2S4. Therefore, these kinds of constructions of hollow p-n heterojunctions have a positive prospect in solar energy conversion fields.


Introduction
The depletion of conventional fossil fuels and the environmental climate issues are forcing people to search for renewable energy [1][2][3][4][5]. Hydrogen, as a renewable energy source, possesses abundant reserves, powerful chemical bonding energy, and environmentally friendly features [6]. For these reasons, photocatalytic hydrogen production from water has attracted widespread attention as an effective way to convert solar energy to clean energy [7]. Honda and Fujishima published ground-breaking research on photoelectrocatalytic water splitting on a TiO 2 electrode in 1974 [8]. Since then, many semiconductor materials have evolved, including metal oxides [9,10], metal sulfides [11], bismuth halides [12,13], and g-C 3 N 4 . [14] As one of the typical n-type semiconductor photocatalytic materials, ZnIn 2 S 4 , benefiting from a suitable band gap (Eg ≈ 2.7 eV) and energy band position, has been widely used in photocatalytic hydrogen production [15]. Furthermore, 2D ZnIn 2 S 4 nanosheets have the advantages of large specific surface areas and abundant surface-active sites, which facilitate involvement in a wide range of photocatalytic redox reactions (for instance, photocatalytic H 2 evolution, Cr VI reduction, and CO 2 conversion) [16]. But the carrier recombination of single ZnIn 2 S 4 nanosheets is still severe. They tend to agglomerate into nanospheres, which vastly reduces their specific surface area and the quantity of active sites, and leads to more disordered carrier migration [17,18]. Previous studies have revealed that coupling ZnIn 2 S 4 with a semiconductor to build a heterojunction can enhance their performance, and mitigate photo corrosion and carrier compounding. Li et al. placed In(OH) 3 in sheet planes along the edges of ZnIn 2 S 4 nanoplates. Under light irradiation, the photocatalytic H 2 evolution performance was about 4.9 times higher compared with pristine ZnIn 2 S 4 , which effectively separates and transfers charges [19]. Liu et al. reported a hybrid photocatalyst prepared by coupling two-dimensional ZnIn 2 S 4 nanosheets with amino-functionalized

Synthesis of ZIF-67
First, 2.91 g of Co(NO 3 ) 2 ·6H 2 O and 3.28 g of 2-methylimidazole were dissolved in 200 mL of methanol solution respectively and then the former was slowly poured into the latter with stirring at room temperature for 24 h. The obtained was centrifuged and washed three times with methanol, and then dried overnight at 60 • in a vacuum oven.

Synthesis of Co 3 O 4
A certain amount of synthesized ZIF-67 was laid flat in a crucible, placed in a muffle furnace, and calcined at a 2 • C/min rate for 2 h under an air atmosphere.

Synthesis of ZnIn 2 S 4 @Co 3 O 4
A certain amount of as-prepared Co 3 O 4 powder was dispersed into the mixture of glycerol (8 mL) and distilled water (32 mL) with the aid of ultrasonication. Then, ZnCl 2 (136.3 mg), C 2 H 5 NS (293.24 mg), and InCl 3 ·4H 2 O (293.24 mg) were added into the mixture and ultrasonicated for 5 min and stirred for 25 min. Next, the obtained suspension was transferred into a 100 mL flask and maintained at 80 • C for 2 h. The resultant solid products

Characterization
The crystallinity of the samples was investigated by X-ray diffraction (XRD) (Bruker D8 Advance; Cu Kα = 1.5404Å,40 kV, 40 mA;) with a scanning rate of 0.05 • /s, using Bragg measurements, and the samples were in powder form, prepared by pressing the slices. The morphologies and sizes of the samples were characterized by SU8010 fieldemission scanning electron microscope (FESEM, Hitachi, Japan) with secondary electron measurements. The beam energy was 10µA, and the samples were fixed with liquid conductive adhesive in deceleration mode with an acceleration voltage of 4.5 kV and a landing voltage of 2.0 kV. Transmission electron microscope (TEM) and high-resolution transmission electron microscopy (HRTEM) were measured on a Talos F200X microscope which operated on an accelerating mode of 200 kV. Samples were prepared by crushing and sieving and then dispersed with solvent onto a copper mesh carbon film. The carbon film was then fixed on the sample rod by clamping the carbon film. The measurement methods include bright field, dark field, and HAADF. X-ray photoelectron spectroscopy (XPS) (Thermo Scientific ESCALAB Xi + ) was carried out to analyze the chemical states of the samples. The X-ray source is monochromic Al Kα radiation (hν = 1486.6 eV). The samples were in powder form. Peak deconvolution and spectral analysis were conducted using xpspeak41 software. The background was chosen as smart and the fitted line pattern was L/G Mix. Linewidth differences were relatively subtle. Semiquantitative analysis of elemental content was performed using the Thermo Scientific ESCALAB Xi+ self-contained database, and a corrected Scofield sensitivity factor. The UV-Vis diffused reflectance spectra (DRS) were recorded using a Cary Series UV-Vis-NIR spectrophotometer (Agilent Technologies). Photoluminescence (PL) measurements were characterized on a Hitachi F-7000 with a 150 W Xe lamp.

Photocatalytic Activities Test
In the photocatalytic activity test system, 20 mg of catalyst was dissolved in a 100 mL aqueous triethanolamine solution (V TEOA :V H2O ) and placed in a 400 mL quartz reactor. Then, nitrogen was infused to remove the air from the inner cavity of the reactor for 30 min upon stirring. After the air was exhausted, the reactor was irradiated with a 300 W Xenon lamp equipped with a 420 nm filter simulating visible light, and circulating water was passed through the outer layer of the reactor to avoid the existence of thermal catalysis. After the hydrogen evolution reaction was performed, the nitrogen and hydrogen mixture gas in the 0.4 mL reactor was taken out by the injection needle and taken every 30 min for a total of six times. Hydrogen content was detected by gas chromatography (FULI 9790) equipped with an AE.5A molecular sieve column (3 m × 3 mm) and the oven temperature, thermal conductivity (detector) temperature, and injector temperature of the gas phase are set to 80 • C, 120 • C, and 100 • C, respectively.

Catalyst Photoelectric Performance Test
In this work, we carried out two photoelectric tests: photocurrent and electrochemical impedance. A three-electrode system and a Chi-760E Chenhua electrochemical workstation are used for the photoelectric test. The three-electrode system used 0.5 mol/L Na 2 SO 4 solution as the electrolyte, platinum electrode as the counter electrode, Ag/AgCl electrode as the reference electrode, and ITO conductive glass coated with a layer of the sample as the working electrode.

Results and Discussion
At room temperature, Zn 2+ and 2-methylimidazole synthesize regular dodecahedral ZIF-67 in a methanol solution, which is subsequently placed in a muffle furnace for cal-Nanomaterials 2023, 13, 758 4 of 13 cination. The organic ligand in ZIF-67 will undergo oxidation, forming hollow, regular dodecahedrons made of Co 3 O 4 particles, and other gases will be liberated. The hollow regular dodecahedron Co 3 O 4 will undergo a half-hour ultrasonic stir with sulfur, indium, zinc, and glycerol, followed by a two-hour immersion in oil at 80 • C in a round-bottled flask. This results in the hollow Co 3 O 4 surface homogeneous load with extremely thin ZnIn 2 S 4 heterojunction material. Figure 1 shows the proposed synthetic method for the p-n heterojunction of ZnIn 2 S 4 @Co 3 O 4 . ZIF-67 was oxidized into a hollow dodecahedral structure composed of Co 3 O 4 particles. Then, the heterojunction material with ultrathin ZnIn 2 S 4 uniformly loaded on the surface of hollow Co 3 O 4 was obtained.
sample as the working electrode.

Results and Discussion
At room temperature, Zn 2+ and 2-methylimidazole synthesize regular dodecahedral ZIF-67 in a methanol solution, which is subsequently placed in a muffle furnace for calcination. The organic ligand in ZIF-67 will undergo oxidation, forming hollow, regular dodecahedrons made of Co3O4 particles, and other gases will be liberated. The hollow regular dodecahedron Co3O4 will undergo a half-hour ultrasonic stir with sulfur, indium, zinc, and glycerol, followed by a two-hour immersion in oil at 80°C in a round-bottled flask. This results in the hollow Co3O4 surface homogeneous load with extremely thin ZnIn2S4 heterojunction material. Figure 1 shows the proposed synthetic method for the p-n heterojunction of ZnIn2S4@Co3O4. ZIF-67 was oxidized into a hollow dodecahedral structure composed of Co3O4 particles. Then, the heterojunction material with ultrathin ZnIn2S4 uniformly loaded on the surface of hollow Co3O4 was obtained. XRD is used to analyze the crystal phase structure of materials. As shown in Figure  2a, the precursor ZIF-67 is effectively synthesized, which is compatible with previously published research [26]. In Figure 2b [15]. For Co3O4@ZIS, the phase of Co3O4 and ZIS are all detected. Compared with the pure sample, they all show the same diffraction peak with no shift, which indicates the successful formation of Co3O4@ZIS heterojunction. XRD is used to analyze the crystal phase structure of materials. As shown in Figure 2a, the precursor ZIF-67 is effectively synthesized, which is compatible with previously published research [26]. In Figure 2b Na2SO4 solution as the electrolyte, platinum electrode as the counter electrode, Ag/AgCl electrode as the reference electrode, and ITO conductive glass coated with a layer of the sample as the working electrode.

Results and Discussion
At room temperature, Zn 2+ and 2-methylimidazole synthesize regular dodecahedral ZIF-67 in a methanol solution, which is subsequently placed in a muffle furnace for calcination. The organic ligand in ZIF-67 will undergo oxidation, forming hollow, regular dodecahedrons made of Co3O4 particles, and other gases will be liberated. The hollow regular dodecahedron Co3O4 will undergo a half-hour ultrasonic stir with sulfur, indium, zinc, and glycerol, followed by a two-hour immersion in oil at 80°C in a round-bottled flask. This results in the hollow Co3O4 surface homogeneous load with extremely thin ZnIn2S4 heterojunction material. Figure 1 shows the proposed synthetic method for the p-n heterojunction of ZnIn2S4@Co3O4. ZIF-67 was oxidized into a hollow dodecahedral structure composed of Co3O4 particles. Then, the heterojunction material with ultrathin ZnIn2S4 uniformly loaded on the surface of hollow Co3O4 was obtained. XRD is used to analyze the crystal phase structure of materials. As shown in Figure  2a, the precursor ZIF-67 is effectively synthesized, which is compatible with previously published research [26]. In Figure 2b  The morphology of the obtained resultant products is examined by FE-SEM. A hollow structure of Co 3 O 4 with a 500 nm size without collapse and fragmentation in Figure 3(a1-a3) was obtained. The spherical ZnIn 2 S 4 comprises many sheets with a diameter of about 1 µm. surfaces is more disordered than that of pure ZnIn 2 S 4 microspheres. The particle sizes of pure ZnIn 2 S 4 flower-like microspheres are quite different from those of all composite samples, the size of pure ZnIn 2 S 4 spheroids is about 1µm, and that of composite Co 3 O 4 (X) @ ZIS is less than 1 µm (Figure 3(e1-e3)). With the increase of the proportion of Co 3 O 4 added in the synthesis process, the particle size of the spherical composite decreases continuously, which probably results from the dispersion of ZnIn 2 S 4 nanosheets with the same mass on more Co 3 O 4 dodecahedron, which causes the decrease in particle size for the spherical composite, indicating the successful construction of heterojunction from the side. low structure of Co3O4 with a 500 nm size without collapse and fragmentation in Figure  3(a1-a3) was obtained. The spherical ZnIn2S4 comprises many sheets with a diameter of about 1 μm. Figure 3(b1-d3) show SEM images of composite samples Co3O4 (10) @ ZIS, Co3O4 (20) @ ZIS and Co3O4 (40) @ ZIS with different proportions, respectively. In terms of surface morphology, these composite samples are spherical, however, the 2D nanosheet arrangement on their surfaces is more disordered than that of pure ZnIn2S4 microspheres. The particle sizes of pure ZnIn2S4 flower-like microspheres are quite different from those of all composite samples, the size of pure ZnIn2S4 spheroids is about 1μm, and that of composite Co3O4 (X) @ ZIS is less than 1 μm (Figure 3(e1-e3)). With the increase of the proportion of Co3O4 added in the synthesis process, the particle size of the spherical composite decreases continuously, which probably results from the dispersion of ZnIn2S4 nanosheets with the same mass on more Co3O4 dodecahedron, which causes the decrease in particle size for the spherical composite, indicating the successful construction of heterojunction from the side.  [27]. Three kinds of lattice fringes of Co3O4 were measured. The lattice distances were 0.284 nm, 0.245 nm, and 0.202 nm corresponding to its three crystal planes (220), (311), and (400), respectively [15]. In addition, the HRTEM image of the edge of the Co3O4 (20) @ ZIS sample shows that two lattice distances (Figure 4h) are 0.323 nm and 0.283 nm, respectively, corresponding to the (102) and (104) crystal planes of ZnIn2S4, which proves that ZnIn2S4 exists in the composite sample. Since Co3O4 is completely covered by ZnIn2S4, the crystal surface of Co3O4 cannot be observed.  [27]. Three kinds of lattice fringes of Co 3 O 4 were measured. The lattice distances were 0.284 nm, 0.245 nm, and 0.202 nm corresponding to its three crystal planes (220), (311), and (400), respectively [15]. In addition, the HRTEM image of the edge of the Co 3 O 4 (20) @ ZIS sample shows that two lattice distances (Figure 4h) are 0.323 nm and 0.283 nm, respectively, corresponding to the (102) and (104) crystal planes of ZnIn 2 S 4 , which proves that ZnIn 2 S 4 exists in the composite sample. Since Co 3 O 4 is completely covered by ZnIn 2 S 4 , the crystal surface of Co 3 O 4 cannot be observed.
To further confirm the intimate contact interface element between Co 3 O 4 and ZnIn 2 S 4 , the HAADF-STEM-EDX was undertaken. Figure 4m demonstrates the uniformly distributed nature of Zn, S, In, Co, and O. From the element distribution of Co and O, it is apparent that Co 3 O 4 is composed of particles, which is consistent with the findings in previous SEM and TEM diagrams. The successful synthesis of the composite sample was further verified.
The X-ray photoelectron spectroscopy (XPS) analysis of the Co 3 O 4 @ ZIS, Co 3 O 4, and ZnIn 2 S 4 composites is shown in Figure 5a. As shown in Figure 5b, the Co 2p spectrum is composed of two spin-orbit doublets and two satellite peaks. The binding energies of the first peak in the composite Co 3 O 4 (20)@ZIS are at 779.70 eV and 781.36 eV, and the second peaks are at 794.28 eV and 796.03 eV, corresponding to the Co 2p 3/2 and Co 2p 1/2 orbitals, respectively, which indicate the coexistence of Co 2+ and Co 3+ [28,29]. The high-resolution XPS spectrum of the O element shows three peaks at 529.65 eV, 531.39 eV, and 532.24 eV in Figure 5c. The first and second characteristic peaks correspond to lattice oxygen in metal oxides and hydroxyl radicals on the surface of materials, while the latter corresponds to adsorbed water on the surface [30][31][32][33][34]. The peaks (Figure 5d) at 1021.73 and 1044.85 eV are attributed to Zn 2p 3/2 and Zn 2p 1/2 , respectively, indicating the presence of Zn 2+ [33]. Figure 5e shows that the two peaks at 444.62 eV and 452.21 eV are assigned to In 3d 5/2 In and In 3d 3/2 respectively, indicating that the chemical state of the cation in the composite is plus three [18]. S 2p spectrum (Figure 5f) can be resolved into two peaks including 162.8 and 161.6 eV, corresponding to the S 2p 1/2 and S 2p 3/2 orbitals of S 2- [34]. Overall, the XPS results indicate that the binding energies of the Zn, In, and S have a level of red shift compared with pure ZnIn 2 S 4 , which confirms the successful construction of the heterojunction [35].  To further confirm the intimate contact interface element between Co3O4 and ZnIn2S4, the HAADF-STEM-EDX was undertaken. Figure 4m demonstrates the uniformly distributed nature of Zn, S, In, Co, and O. From the element distribution of Co and O, it is apparent that Co3O4 is composed of particles, which is consistent with the findings in previous SEM and TEM diagrams. The successful synthesis of the composite sample was further verified.
The X-ray photoelectron spectroscopy (XPS) analysis of the Co3O4 @ ZIS, Co3O4, and ZnIn2S4 composites is shown in Figure 5a. As shown in Figure 5b, the Co 2p spectrum is composed of two spin-orbit doublets and two satellite peaks. The binding energies of the first peak in the composite Co3O4(20)@ZIS are at 779.70 eV and 781.36 eV, and the second peaks are at 794.28 eV and 796.03 eV, corresponding to the Co 2p3/2 and Co 2p1/2 orbitals, respectively, which indicate the coexistence of Co 2+ and Co 3+ [28,29]. The high-resolution XPS spectrum of the O element shows three peaks at 529.65 eV, 531.39 eV, and 532.24 eV in Figure 5c. The first and second characteristic peaks correspond to lattice oxygen in metal oxides and hydroxyl radicals on the surface of materials, while the latter corresponds to adsorbed water on the surface [30][31][32][33][34]. The peaks (Figure 5d) at 1021.73 and 1044.85 eV are attributed to Zn 2p3/2 and Zn 2p1/2, respectively, indicating the presence of Zn 2+ [33]. Figure 5e shows that the two peaks at 444.62 eV and 452.21 eV are assigned to In 3d5/2In and In 3d3/2 respectively, indicating that the chemical state of the cation in the composite is plus three [18]. S 2p spectrum (Figure 5f) can be resolved into two peaks including 162.8 and 161.6 eV, corresponding to the S 2p1/2 and S 2p3/2 orbitals of S 2- [34]. Overall, the XPS results indicate that the binding energies of the Zn, In, and S have a level The samples' nitrogen adsorption-desorption and pore size distribution curves are shown in Figure 6a-c. Naturally, type IV isotherm and type H3 hysteresis rings are found in all samples, demonstrating the presence of capillary condensation. The materials' microstructure and specific surface area are typically closely connected. As can be observed from the little figure, the three samples all have pore sizes that are mostly spread between The samples' nitrogen adsorption-desorption and pore size distribution curves are shown in Figure 6a-c. Naturally, type IV isotherm and type H3 hysteresis rings are found in all samples, demonstrating the presence of capillary condensation. The materials' microstructure and specific surface area are typically closely connected. As can be observed from the little figure, the three samples all have pore sizes that are mostly spread between 1 and 75 nm, which indicates that they are all mesoporous materials. In Table 1, Co 3 O 4 (20)@ZIS, ZnIn 2 S 4 , and Co 3 O 4 had specific surface area distributions of 121, 15, and 71 m 2 g −1 , respectively. Evidently, the specific surface area and pore volume of the Co 3 O 4 (20)@ZIS composite samples increased with the addition of ZnIn 2 S 4 in comparison to pure Co 3 O 4 . It is possible that the composite catalyst will offer more active centers, which will make it easier for photocatalytic hydrogen evolution events to occur. Another angle is that the success of the catalyst preparation is demonstrated by the modification of the adsorption characteristics of the composite samples. The samples' nitrogen adsorption-desorption and pore size distribution curves are shown in Figure 6a-c. Naturally, type IV isotherm and type H3 hysteresis rings are found in all samples, demonstrating the presence of capillary condensation. The materials' microstructure and specific surface area are typically closely connected. As can be observed from the little figure, the three samples all have pore sizes that are mostly spread between 1 and 75 nm, which indicates that they are all mesoporous materials. In Table 1, Co3O4(20)@ZIS, ZnIn2S4, and Co3O4 had specific surface area distributions of 121, 15, and 71 m 2 g −1 , respectively. Evidently, the specific surface area and pore volume of the Co3O4(20)@ZIS composite samples increased with the addition of ZnIn2S4 in comparison to pure Co3O4. It is possible that the composite catalyst will offer more active centers, which will make it easier for photocatalytic hydrogen evolution events to occur. Another angle is that the success of the catalyst preparation is demonstrated by the modification of the adsorption characteristics of the composite samples.   The characterization of phase, morphology, and structure proves the successful synthesis of hollow structure Co3O4, and ZnIn2S4 successfully grows on the surface of Co3O4. Based on this point, the hydrogen production performance of all samples is tested under visible light in Figure 7a. Co3O4 (20) @ ZIS represents the best ration of the composite sample. Its hydrogen production is as high as 16.14 mmol g −1 in three hours, while the hydro-  The characterization of phase, morphology, and structure proves the successful synthesis of hollow structure Co 3 O 4 , and ZnIn 2 S 4 successfully grows on the surface of Co 3 O 4 . Based on this point, the hydrogen production performance of all samples is tested under visible light in Figure 7a. Co 3 O 4 (20) @ ZIS represents the best ration of the composite sample. Its hydrogen production is as high as 16.14 mmol g −1 in three hours, while the hydrogen production performance of pure ZnIn 2 S 4 is only 2.22 mmol g −1 . In addition, the hydrogen production of Co 3 O 4 is almost 0 mmol g −1 , which may be due to the narrow band gap of Co 3 O 4 , which causes the carriers to recombine rapidly and prevents them from moving to the surface to take part in the proton reduction process. Figure 7b clearly shows the hydrogen production rate of samples with different proportions, the Co 3 O 4 (20) @ ZIS with the best proportion reaches 5.38 mmol g −1 L −1 . In addition, the hydrogen production rate first rises and then falls with the increase of the proportion of ZnIn 2 S 4 in the composite sample, which may be due to too much ZnIn 2 S 4 being loaded onto the surface of the Co 3 O 4 , thus making the active sites on Co 3 O 4 not fully exposed, and resulting in the hole not reacting with sacrificial agents in time, the increase of carrier recombination efficiency, and the decrease of hydrogen production performance [36]. Subsequently, the hydrogen production of Co 3 O 4 (20) @ ZIS is tested for a long time to test the stability of the sample. Meanwhile, as shown in Figure 7c and Figure S2, the hydrogen production rate is essentially constant within 12 h, proving the stability of the sample. It also explains that the loading of Co 3 O 4 enhances the photocatalytic stability of ZnIn 2 S 4 . Co 3 O 4 effectively captures the photo-generated holes produced by the photoexcitation of ZnIn 2 S 4 , which significantly reduces the oxidation process of S 2− by holes, and effectively alleviates the photo-corrosion problem of ZnIn 2 S 4 . To find out the potential reasons for the improved performance of Co3O4 (X) @ ZIS solid-ultraviolet diffuse reflectance (DRS) test, photoluminescence emission spectru (PL), photocurrent, electrochemical impedance, and other photoelectrochemical tests carried out to evaluate the photoelectric performance, carrier separation, and transfer ficiency of the samples. Figure 8a is the solid-ultraviolet diffuse reflectance spectrum (U VIS DRS). The absorption range of ZnIn2S4 is about 510 nm, and Co3O4 absorbs in wavelength range of ultraviolet, visible, and near-infrared light. With the increase of proportion of Co3O4, the absorption range of the composite sample is red-shifted grad ally, which corresponds to the color variation of the sample, indicating that ZnIn2S uniformly loaded on the surface of the Co3O4 dodecahedron with a hollow structure a the existence of black Co3O4 increases the absorption range and intensity of the samp According to the formula: The energy band gaps of Co3O4 and ZnIn2S4 are calculated to be 1.32 eV and 2.70 based on the optical absorption band. This result will be helpful to the later energy ba mechanism. To find out the potential reasons for the improved performance of Co 3 O 4 (X) @ ZIS, a solid-ultraviolet diffuse reflectance (DRS) test, photoluminescence emission spectrum (PL), photocurrent, electrochemical impedance, and other photoelectrochemical tests are carried out to evaluate the photoelectric performance, carrier separation, and transfer efficiency of the samples. Figure 8a is the solid-ultraviolet diffuse reflectance spectrum (UV-VIS DRS). The absorption range of ZnIn 2 S 4 is about 510 nm, and Co 3 O 4 absorbs in the wavelength range of ultraviolet, visible, and near-infrared light. With the increase of the proportion of Co 3 O 4 , the absorption range of the composite sample is red-shifted gradually, which corresponds to the color variation of the sample, indicating that ZnIn 2 S 4 is uniformly loaded on the surface of the Co 3 O 4 dodecahedron with a hollow structure and the existence of black Co 3 O 4 increases the absorption range and intensity of the sample. According to the formula: Mott-Schottky tests were performed to analyze the energy band structure of Co3O4 and ZnIn2S4, and reasonable assumptions were made on the catalytic mechanism of catalyst materials based on the energy band structure (Figure 9). According to the formula: The Mott-Schottky curve of ZnIn2S4, in which the slope of the linear part is positive, indicates that ZnIn2S4 is an n-type semiconductor, and the flat band potential (Efb) is −0.54 eV (−0.76 eV vs. Ag/AgCl), relative to the standard hydrogen electrode. Figure 9b shows the Mott-Schottky curve of Co3O4, in which the slope of the linear part is negative, indicating that Co3O4 is a p-type semiconductor, and the flat band potential (Efb) is 1.92 eV (1.70 vs. Ag/AgCl) relative to the standard hydrogen electrode. According to earlier findings, the conduction band value for n-type semiconductors is 0.1 eV lower than the flatband potential, while the valence band value for p-type semiconductors is 0.1 eV higher than the flat-band potential, resulting in ECB (ZnIn2S4) =−0.64 eV and EVB (Co3O4) = 2.02 eV. In conjunction with the band gap values determined using the DRS pattern from the prior research, as shown in Figure 9c, Eg (ZnIn2S4) = 2.70 eV and Eg (Co3O4) = 1.32 eV, it can be calculated that EVB (ZnIn2S4) =−2.06 eV and ECB (Co3O4) = 0.70 eV. This energy band structure promotes the formation of Co3O4 (X) @ ZIS p-n heterojunction.
The mechanism hypothesis of Co3O4 (X) @ ZnIn2S4 heterojunction participating in photocatalytic hydrogen production is shown in Figure 10. Before the Co3O4 and ZnIn2S4 contact, ZnIn2S4 has a higher Fermi level (Ef) than Co3O4. When ZnIn2S4 is loaded on Co3O4, a close contact surface will be formed between them. The negative charge in ZnIn2S4 is transferred to Co3O4 through the interface, and the positive charge in Co3O4 is transferred to ZnIn2S4 through the interface until their Fermi levels are equal, and a builtin electric field from ZnIn2S4 to Co3O4 is formed. It is worth mentioning that the energy band positions of the two will also move with the Fermi level, and the conduction band of Co3O4 will move to a more negative position than that of ZnIn2S4 so that electrons on the Co3O4 conduction band can transfer to ZnIn2S4 conduction band. However, the builtin electric field can further guide the electron transfer from the conduction band of p-type Transient photocurrent response (TPR) in a classical three-electrode system is used to continue the exploration of the photoelectrochemical characteristics of Co 3 O 4 (X) @ ZIS, to understand the potential reasons for the enhanced carrier separating and transfer ability of Co 3 O 4 (X) @ ZIS. As shown in Figure 8c, since the black Co 3 O 4 has a narrow band gap (Eg = 1.32 eV) and belongs to a narrow band gap transition metal semiconductor, the carrier recombination of Co 3 O 4 is too fast, resulting in no photocurrent signal or its weak signal. Meanwhile, it is found that pure ZnIn 2 S 4 nanospheres exhibit a small photocurrent intensity due to the corrosion of ternary sulfide and serious carrier recombination caused by the solid structure. As anticipated, after loading ZnIn 2 S 4 onto the hollow nanostructure of Co 3 O 4 , the photocurrent intensity of the composite sample was significantly higher than that of the two substrates. Among all the composite samples, Co 3 O 4 (20) @ ZIS has the strongest photocurrent signal, indicating that Co 3 O 4 in the heterojunction system can make ZnIn 2 S 4 carriers excited under visible light separate and migrate rapidly. The transfer impedance of photoexcited carriers in the heterojunction is observed via electrochemical impedance spectroscopy (EIS). As shown in Figure 8d, the semicircle formed by the Nernst curve of Co 3 O 4 (20) @ ZIS is the smallest semicircle in the produced samples, offering the lowest transfer impedance during photocatalytic activity [37,38]. It is in perfect agreement with the photocurrent test and fluorescence test.
Mott-Schottky tests were performed to analyze the energy band structure of Co 3 O 4 and ZnIn 2 S 4 , and reasonable assumptions were made on the catalytic mechanism of catalyst materials based on the energy band structure (Figure 9). According to the formula: Nanomaterials 2023, 13, x FOR PEER REVIEW 11 of 13 semiconductor Co3O4 to the conduction band of ZnIn2S4, and the hole transfer from the valence band of n-ZnIn2S4 to the valence band of Co3O4, which promotes the spatial separation of electron-hole pairs. Therefore, more electrons are transferred to ZnIn2S4 nanosheets and participate in the proton reduction reaction.

Conclusions
The nanocomposite Co3O4@ ZnIn2S4 with intimate contact was synthesized by the mild oil bath. The p-n heterojunctions were successfully constructed, forming the internal electric field and effectively realizing spatial separation of carriers. Moreover, Co3O4 broadened the absorbance range and enhanced the absorbance intensity of the composite The Mott-Schottky curve of ZnIn 2 S 4 , in which the slope of the linear part is positive, indicates that ZnIn 2 S 4 is an n-type semiconductor, and the flat band potential (E fb ) is −0.54 eV (−0.76 eV vs. Ag/AgCl), relative to the standard hydrogen electrode. Figure 9b shows the Mott-Schottky curve of Co 3 O 4 , in which the slope of the linear part is negative, indicating that Co 3 O 4 is a p-type semiconductor, and the flat band potential (E fb ) is 1.92 eV (1.70 vs. Ag/AgCl) relative to the standard hydrogen electrode. According to earlier findings, the conduction band value for n-type semiconductors is 0.1 eV lower than the flat-band potential, while the valence band value for p-type semiconductors is 0.1 eV higher than the flat-band potential, resulting in ECB (ZnIn 2 S 4 ) =−0.64 eV and EVB (Co 3 O 4 ) = 2.02 eV. In conjunction with the band gap values determined using the DRS pattern from the prior research, as shown in Figure 9c, Eg (ZnIn 2 S 4 ) = 2.70 eV and Eg (Co 3 O 4 ) = 1.32 eV, it can be calculated that EVB (ZnIn 2 S 4 ) =−2.06 eV and ECB (Co 3 O 4 ) = 0.70 eV. This energy band structure promotes the formation of Co 3 O 4 (X) @ ZIS p-n heterojunction.
The mechanism hypothesis of Co 3 O 4 (X) @ ZnIn 2 S 4 heterojunction participating in photocatalytic hydrogen production is shown in Figure 10. Before the Co 3 O 4 and ZnIn 2 S 4 contact, ZnIn 2 S 4 has a higher Fermi level (Ef) than Co 3 O 4 . When ZnIn 2 S 4 is loaded on Co 3 O 4 , a close contact surface will be formed between them. The negative charge in ZnIn 2 S 4 is transferred to Co 3 O 4 through the interface, and the positive charge in Co 3 O 4 is transferred to ZnIn 2 S 4 through the interface until their Fermi levels are equal, and a built-in electric field from ZnIn 2 S 4 to Co 3 O 4 is formed. It is worth mentioning that the energy band positions of the two will also move with the Fermi level, and the conduction band of Co 3 O 4 will move to a more negative position than that of ZnIn 2 S 4 so that electrons on the Co 3 O 4 conduction band can transfer to ZnIn 2 S 4 conduction band. However, the built-in electric field can further guide the electron transfer from the conduction band of p-type semiconductor Co 3 O 4 to the conduction band of ZnIn 2 S 4 , and the hole transfer from the valence band of n-ZnIn 2 S 4 to the valence band of Co 3 O 4 , which promotes the spatial separation of electron-hole pairs. Therefore, more electrons are transferred to ZnIn 2 S 4 nanosheets and participate in the proton reduction reaction. Figure 9. (a,b) The Mott-Schottky curve of ZnIn2S4 and Co3O4;(c) The band gap of ZnIn2S4 and Co3O4. Figure 10. Mechanism of photocatalytic hydrogen production from Co3O4(X)@ZnIn2S4 p-n heterojunction.

Conclusions
The nanocomposite Co3O4@ ZnIn2S4 with intimate contact was synthesized by the mild oil bath. The p-n heterojunctions were successfully constructed, forming the internal electric field and effectively realizing spatial separation of carriers. Moreover, Co3O4 broadened the absorbance range and enhanced the absorbance intensity of the composite Co3O4@ ZnIn2S4 in this hybridized system. Furthermore, the hollow structure of Co3O4 further improved the utilization rate of light. As a result, the optimal Co3O4 (20)@ ZnIn2S4 photocatalyst exhibited an H2 evolution rate of 5.38 mmol·g −1 ·L −1 . this rate is seven times Figure 10. Mechanism of photocatalytic hydrogen production from Co 3 O 4 (X)@ZnIn 2 S 4 p-n heterojunction.

Conclusions
The nanocomposite Co 3 O 4 @ ZnIn 2 S 4 with intimate contact was synthesized by the mild oil bath. The p-n heterojunctions were successfully constructed, forming the internal electric field and effectively realizing spatial separation of carriers. Moreover, Co 3 O 4 broadened the absorbance range and enhanced the absorbance intensity of the composite Co 3 O 4 @ ZnIn 2 S 4 in this hybridized system. Furthermore, the hollow structure of Co 3 O 4 further improved the utilization rate of light. As a result, the optimal Co 3 O 4 (20)@ ZnIn 2 S 4 photocatalyst exhibited an H 2 evolution rate of 5.38 mmol·g −1 ·L −1 . this rate is seven times higher than that of pure ZnIn 2 S 4 . Therefore, this work offers some helpful advice for the development of hollow p-n heterojunctions in the future and their utilization in the area of energy conversion.